1. Field of the Invention
This invention relates to amplifiers and, more particularly, to a nested feed forward distortion reduction system for amplifiers.
2. Description of Related Art
Amplifiers often add undesired distortion to a signal, creating an output signal comprising distortion or nonlinear components and the input signal component. The distortion includes any undesired signals added to or affecting adversely the signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modem amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of the input signal pattern as well as the resultant distortion location, some feed forward schemes inject a known signal, i.e. a pilot signal, in the main signal path to go with the distortion produced by the amplification process. By designing the feed forward distortion reduction circuitry to detect and reduce the pilot signal, the distortion is also reduced.
FIG. 1 discloses a feed forward correction circuitry 10 which can use a pilot signal to reduce distortion produced by RF amplifier 12. A signal, such as a carrier signal, is applied to a splitter 14. The splitter 14 replicates or produces an analog representation of the signal on a main signal path 16 and a feed forward path 18. The splitter 14 is part of a feed forward loop referred to as loop # 1, which in addition to the splitter 14, comprises a gain & phase circuit 20, coupler 22, the RF amplifier 12, delay circuit 24 and couplers 26 and 28. The signal on the main path 16 is applied to gain & phase circuit 20. The output of gain & phase circuit 20 and the pilot signal are applied to the coupler 22. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the signal so as not to interfere with the operation of the amplifier 12. The output of the coupler 22 is applied to the amplifier 12 whose output comprises the amplified signal, the amplified pilot signal and distortion signals produced by the amplifier 12. A portion of the output of the amplifier 12 is obtained from the coupler 26 and is combined with a delayed version of the signal (signal on path 18) at the coupler 28 via coupling path 30. The signal on the path 18 has experienced sufficient delay provided by delay circuit 24 so that such signal experiences the same delay as the signal appearing at the coupler 28 via the path 30.
The gain & phase circuit 20 is controlled via control path 32 with control signals to adjust the gain and phase of the signal such that the signal appearing at the coupler 28 via the path 30 is substantially the inverse (equal in amplitude but 180.degree. out of phase) of the delayed signal at the coupler 28. The control signal appearing on the control path 32 of the gain & phase circuit 20 is derived from the signal at point A in a well known manner such as the use of detection circuits. The detection circuits detect well known electrical signal characteristics such as amplitude, phase, and frequency of the signal. Therefore, the signals applied to the coupler 28 substantially cancel each other leaving at point A the pilot signal and the distortion produced by the amplifier 12. Loop # 1 is thus a feed forward loop which serves to isolate at point A the pilot signal and distortion signals produced by the amplifier 12.
The signals appearing at point A (pilot signal and distortion signals) are fed to gain & phase circuit 34 whose output is fed to main correction amplifier 36 whose output is applied to coupler 38. A portion of the output signals (signal, pilot signal and distortion signals) of the amplifier 12 is fed to delay circuit 40 whose output is fed to the coupler 38. The delay circuit 40 is designed such that signals from the output of the amplifier 12 applied to the coupler 38 experience substantially the same delay as the signals from the output of the amplifier 36 applied to the coupler 38.
The pilot signal is used to obtain information about how well the distortion is being cancelled from the main signal path 16. The information is obtained by detecting well known electrical signal characteristics of the pilot signal, such as the amplitude, spectral content, phase response of the pilot signal. For example, the amplitude of the pilot signal after cancellation at the coupler 38 can indicate how well the distortion is being cancelled. If the amplitude of the pilot signal is small after the coupler 38, the amplitude of the distortion is also small. Detection circuit 42, such as a mixer connected to a log detector (or other known detection circuits), will detect the pilot signal and use this information to generate control signals onto path 46 to cause the gain & phase circuit 34 to modify the pilot signal at point A such that the pilot signal on the main path 16 at the coupler 38 is substantially the inverse (equal in amplitude but 180.degree. out of phase) of the pilot signal on the feed forward path 18 at the coupler 38. The corresponding pilot signals and the distortion signals at the coupler 38 substantially cancel each other respectively at the coupler 38 leaving the signal (or an amplified version of the signal) at the output of the system. Therefore, loop # 2, which comprises the coupler 26, the coupler 28, the gain & phase circuit 34, the amplifier 36, the coupler 38 and the delay circuit 40 is a feed forward loop which uses the information obtained from the pilot signal to cancel substantially the distortion produced by the amplifier 12.
In actual systems, however, there is rarely an absolute cancellation of the distortion and the pilot signals. Feed forward distortion reduction systems require tight operating tolerances, for example, to achieve a 30 dB reduction in IMDs, typical feed forward correction systems may require a + or -0.1 dB frequency flat response (amplitude deviation over the frequency band of operation) and a + or -1 degree phase linearity (phase deviation from a straight line in the frequency band of operation). To obtain this accuracy is difficult. In feed forward distortion reduction signals which use a pilot signal, the amplitude of the pilot signal is typically relatively small at the output of the feed forward distortion reduction system because of the cancellation of the pilot and the relative amplitude of the pilot signal with respect to the amplitude of the output signal. Thus, it becomes difficult to detect the pilot signal at the output of the system. To improve detection of the pilot signal at the output of the distortion reduction system, schemes are developed to generate the pilot signal at an appropriate location and to improve detection and control. Such schemes typically add costs to the systems.
Pilotless feed forward distortion reduction schemes have been developed to eliminate the pilot signal, thereby eliminating the need for the pilot generation, detection and control circuitry, such as the coupler 22 and pilot detection circuit 42. The pilotless feed forward reduction systems, however, do not have a known pilot signal which can be detected at the output of the feed forward distortion reduction system to compensate for changing operating conditions. Instead of detecting the pilot signal to improve cancellation at the coupler 38, the pilotless feed forward systems can use gain and phase control circuitry 54 responsive to the inputs from the couplers 56 and 58 to produce gain and phase control signals to the gain and phase circuits 34. In response, the gain and phase circuits 34 provide variable phase and/or gain adjustments which maintain the appropriate gain and/or phase for the distortion on the feed forward path 18 to improve reduction of the distortion of the main signal path 16 at the coupler 38. The gain and phase control circuitry adds costs and complexity, and to achieve adequate reduction of the distortion is difficult. For example, the correction amplifier 36 produces the amplified distortion with second distortion which is injected into the main signal path 16.
A need exists for a distortion reduction system that can provide adequate distortion reduction while reducing any problems associated with other distortion reduction systems.